David Cameron is to meet Russian president Vladimir Putin for talks on the Syrian crisis amid fears that differences between Moscow and the West are pushing the two sides towards a new Cold War.
The meeting, on the eve of the G8 summit in Lough Erne, comes as President Barack Obama is preparing to arm the rebels, saying the US administration was convinced the Syrian regime had used chemical weapons against its own people.
Mr Putin has made no secret of his opposition to any move by the West to supply arms to the Syrian opposition. He responded to the recent lifting of the EU arms embargo by reaffirming his intention to supply president Bashar al-Assad with sophisticated anti-aircraft missiles in order to deter "hotheads" from intervening in the conflict.
The Russian president has also reacted sceptically to evidence produced by Britain, France and the US that the regime has used chemical weapons - crossing Mr Obama's "red line" for intervention.
Mr Cameron has welcomed Mr Obama's announcement, although he has yet to decide whether to follow the president's lead on supplying arms to the rebels. However he said Britain would continue to offer non-lethal support to what he called the "genuine" opposition - saying it was vital to bolster the democratic elements against the extremists.
"Yes there are elements of the Syrian opposition that are deeply unsavoury, that are very dangerous, very extremist and I want nothing to do with them. I'd like them driven out of Syria - they're linked to al Qaida," he said in an interview with Sky News.
"But there are elements of the Syrian opposition who want to see a free democratic, pluralistic Syria that respects the rights of minorities including Christians and we should be working with them - we are working with them.
"If we don't work with those elements of the Syrian opposition, then we can't be surprised if the only elements of the Syrian opposition that are getting, that are actually making any progress in Syria, are the ones that we don't approve of. After all, they are trying to defend their communities against appalling attacks, including, let's be clear, chemical weapon attacks."
Even if Mr Cameron were to decide that Britain should start supplying arms, he may find his path blocked in the face of strong opposition in Parliament across all three main parties.
The Prime Minister again reaffirmed the commitment wrung out of him by Tory rebels that he would give MPs a vote if he decided Britain should go ahead and arm the Syrian opposition. Liberal Democrat Cabinet minister Danny Alexander indicated that his party was likely to be wary about arming the rebels.